linux源代码剖析之二boot

“boot”
boot.s
|
| boot.s
|
| boot.s is loaded at 0x7c00 by the bios-startup routines, and moves itself
| out of the way to address 0x90000, and jumps there.
|
| It then loads the system at 0x10000, using BIOS interrupts. Thereafter
| it disables all interrupts, moves the system down to 0x0000, changes
| to protected mode, and calls the start of system. System then must
| RE-initialize the protected mode in it’s own tables, and enable
| interrupts as needed.
|
| NOTE! currently system is at most 8*65536 bytes long. This should be no
| problem, even in the future. I want to keep it simple. This 512 kB
| kernel size should be enough - in fact more would mean we’d have to move
| not just these start-up routines, but also do something about the cache-
| memory (block IO devices). The area left over in the lower 640 kB is meant
| for these. No other memory is assumed to be “physical”, ie all memory
| over 1Mb is demand-paging. All addresses under 1Mb are guaranteed to match
| their physical addresses.
|
| NOTE1 abouve is no longer valid in it’s entirety. cache-memory is allocated
| above the 1Mb mark as well as below. Otherwise it is mainly correct.
|
| NOTE 2! The boot disk type must be set at compile-time, by setting
| the following equ. Having the boot-up procedure hunt for the right
| disk type is severe brain-damage.
| The loader has been made as simple as possible (had to, to get it
| in 512 bytes with the code to move to protected mode), and continuos
| read errors will result in a unbreakable loop. Reboot by hand. It
| loads pretty fast by getting whole sectors at a time whenever possible.

.globl begtext, begdata, begbss, endtext, enddata, endbss
.text
begtext:
.data
begdata:
.bss
begbss:
.text

BOOTSEG = 0x07c0
INITSEG = 0x9000
SYSSEG = 0x1000 | system loaded at 0x10000 (65536).
ENDSEG = SYSSEG + SYSSIZE

entry start
start:
mov ax,#BOOTSEG
mov ds,ax
mov ax,#INITSEG
mov es,ax
mov cx,#256
sub si,si
sub di,di
rep
movw
jmpi go,INITSEG
go: mov ax,cs
mov ds,ax
mov es,ax
mov ss,ax
mov sp,#0x400 | arbitrary value >>512

mov  ah,#0x03    | read cursor pos
xor bh,bh
int 0x10mov cx,#24
mov bx,#0x0007  | page 0, attribute 7 (normal)
mov bp,#msg1
mov ax,#0x1301  | write string, move cursor
int 0x10

| ok, we’ve written the message, now
| we want to load the system (at 0x10000)

mov  ax,#SYSSEG
mov es,ax       | segment of 0x010000
call    read_it
call    kill_motor

| if the read went well we get current cursor position ans save it for
| posterity.

mov  ah,#0x03    | read cursor pos
xor bh,bh
int 0x10        | save it in known place, con_init fetches
mov [510],dx    | it from 0x90510.

| now we want to move to protected mode …

cli          | no interrupts allowed !

| first we move the system to it’s rightful place

mov  ax,#0x0000
cld         | 'direction'=0, movs moves forward

do_move:
mov es,ax | destination segment
add ax,#0x1000
cmp ax,#0x9000
jz end_move
mov ds,ax | source segment
sub di,di
sub si,si
mov cx,#0x8000
rep
movsw
j do_move

| then we load the segment descriptors

end_move:

mov  ax,cs       | right, forgot this at first. didn't work :-)
mov ds,ax
lidt    idt_48      | load idt with 0,0
lgdt    gdt_48      | load gdt with whatever appropriate

| that was painless, now we enable A20

call empty_8042
mov al,#0xD1        | command write
out #0x64,al
call    empty_8042
mov al,#0xDF        | A20 on
out #0x60,al
call    empty_8042

| well, that went ok, I hope. Now we have to reprogram the interrupts

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